Transcript IEEE 802.11 Overview - University of California, Berkeley

IEEE 802.11 Overview
Mustafa Ergen
[email protected]
UC Berkeley
Wireless Market Segments
Wireless Market Segments & Partners
Wireless Internetworking
Overview
Residential/
Premise/ Campus
Fixed
Mobile
Broadband Multiservice
IEEE
802.11
BLUE
TOOTH
MMDS
LMDS
Cisco/
Bosch
2G+
Cellular
3G
Cellular
Data
Services
Packet
Data/Voice
GPRS
Mobile IP
UMTS
Standardization of Wireless
Networks


Wireless networks are standardized by IEEE.
Under 802 LAN MAN standards committee.
ISO
OSI
7-layer
model
Application
Presentation
Session
Transport
Network
Logical Link Control
Data Link
Medium Access (MAC)
Physical
IEEE 802
standards
Physical (PHY)
IEEE 802.11 Overview
Adopted in 1997.
Defines;
 MAC sublayer
 MAC management
protocols and services
 Physical (PHY) layers
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IR
FHSS
DSSS
Goals
•To deliver services in wired networks
•To achieve high throughput
•To achieve highly reliable data delivery
•To achieve continuous network connection.
Components
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Station
BSS - Basic Service Set
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ESS - Extended Service Set
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IBSS : Infrastructure BSS : QBSS
A set of infrastrucute BSSs.
Connection of APs
Tracking of mobility
DS – Distribution System
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AP communicates with another
Services
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Station services:
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authentication,
de-authentication,
privacy,
delivery of data
Distribution Services ( A thin layer between MAC and LLC sublayer)
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association
disassociation
reassociation
distribution
Integration
A station maintain two variables:
• authentication state (=> 1)
• association state
(<= 1)
Ex.
Medium Access Control
Functionality;
 Reliable data delivery
 Fairly control access
 Protection of data
Deals;
 Noisy and unreliable medium
 Frame exchange protocol - ACK
 Overhead to IEEE 802.3  Hidden Node Problem – RTS/CTS
 Participation of all stations
 Reaction to every frame
MAC
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Retry Counters
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Basic Access Mechanism
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Short retry counter
Long retry counter
Lifetime timer
CSMA/CA
Binary exponential back-off
NAV – Network Allocation Vector
Timing Intervals: SIFS, Slot Time, PIFS, DIFS, EIFS
DCF Operation
PCF Operation
DCF Operation
PCF Operation
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Poll – eliminates contention
PC – Point Coordinator
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CFP – Contention Free Period
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Polling List
Over DCF
PIFS
Alternate with DCF
Periodic Beacon – contains length of CFP
CF-Poll – Contention Free Poll
NAV prevents during CFP
CF-End – resets NAV
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Frame Types
NAV information
Or
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FC
2
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Upper layer data
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2048 byte max
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256 upper layer
header
Short Id for PSPoll
Duration Address Address Address Sequence Address
/ID
1
2
3
Control
4
2
Protocol Version
Frame Type and
Sub Type
To DS and From
DS
More Fragments
Retry
Power
Management
More Data
WEP
Order
6
6
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6
IEEE 48 bit
address
Individual/Group
Universal/Local
46 bit address
BSSID –BSS
Identifier
TA - Transmitter
RA - Receiver
SA - Source
DA - Destination
2
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MSDU
Sequence
Number
Fragment
Number
6
DATA
0-2312
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FCS
4
CCIT CRC-32
Polynomial
bytes
Frame Subtypes
CONTROL
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RTS
CTS
ACK
PS-Poll
CF-End & CF-End
ACK
DATA
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Data
Data+CF-ACK
Data+CF-Poll
Data+CF-ACK+CFPoll
Null Function
CF-ACK (nodata)
CF-Poll (nodata)
CF-ACK+CF+Poll
MANAGEMENT
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Beacon
Probe Request & Response
Authentication
Deauthentication
Association Request &
Response
Reassociation Request &
Response
Disassociation
Announcement Traffic
Indication Message (ATIM)
Other MAC Operations
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Fragmentation
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Sequence control field
In burst
Medium is reserved
NAV is updated by ACK
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WEP Details
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Privacy
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WEP bit set when encrypted.
Only the frame body.
Medium is reserved
NAV is updated by ACK
Symmetric variable key
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Two mechanism
 Default keys
 Key mapping
WEP header and trailer
 KEYID in header
 ICV in trailer
dot11UndecryptableCount
 Indicates an attack.
dot11ICVErrorCount
 Attack to determine a
key is in progress.
MAC Management
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Interference by users that have no concept of data
communication. Ex: Microwave
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Interference by other WLANs
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Security of data
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Mobility
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Power Management
Authentication
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Authentication
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Prove identity to another
station.
Open system authentication
Shared key authentication
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A sends
B responds with a text
A encrypt and send back
B decrypts and returns an
authentication
management frame.
May authenticate any
number of station.
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Security Problem
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A rogue AP
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SSID of ESS
Announce its presence
with beaconing
A active rogue reach
higher layer data if
unencrypted.
Association
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Association
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Transparent mobility
After authentication
Association request to an AP
After established, forward data
To BSS, if DA is in the BSS.
To DS, if DA is outside the BSS.
To AP, if DA is in another BSS.
To “portal”, if DC is outside the ESS.
Portal : transfer point : track mobility. (AP, bridge, or router) transfer 802.1h
New AP after reassociation, communicates with the old AP.
Address Filtering
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More than one WLAN
Three Addresses
Receiver examine the
DA, BSSID
Privacy MAC Function
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WEP Mechanism
Power Management
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Independent BSS
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Overhead
Distributed
 Sender
Data frame handshake
 Announcement
Wake up every beacon.
frame
 Buffer
Awake a period of ATIM after each
 Power
beacon.
consumption in
Send ACK if receive ATIM frame &
ATIM
awake until the end of next ATIM.
 Receiver
 Awake for every
Estimate the power saving station,
Beacon and ATIM
and delay until the next ATIM.
Multicast frame : No ACK : optional
Power Management
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Infrastructure BSS
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Centralized in the AP.
Greater power saving
Mobile Station sleeps for a
number of beacon periods.
Awake for multicast indicated in
DTIM in Beacon.
AP buffer, indicate in TIM
Mobile requests by PS-Poll
Synchronization
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Timer Synchronization in an Infrastructure BSS
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Timer Synchronization in an IBSS
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Beacon contains TSF
Station updates its with the TSF in beacon.
Distributed. Starter of the BSS send TSF zero and increments.
Each Station sends a Beacon
Station updates if the TSF is bigger.
Small number of stations: the fastest timer value
Large number of stations: slower timer value due to collision.
Synchronization with Frequency Hopping PHY Layers
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Changes in a frequency hopping PHY layer occurs periodically (the dwell
meriod).
Change to new channel when the TSF timer value, modulo the dwell period,
is zero
Scanning & Joining
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Scanning
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Passive Scanning : only listens for Beacon and get
info of the BSS. Power is saved.
Active Scanning: transmit and elicit response from
APs. If IBSS, last station that transmitted beacon
responds. Time is saved.
Joining a BSS
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Syncronization in TSF and frequency : Adopt PHY
parameters : The BSSID : WEP : Beacon Period :
DTIM
Combining Management Tools
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Combine Power Saving Periods with Scanning
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Instead of entering power saving mode, perform
active scanning.
Gather information about its environments.
Preauthentication
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Scans and initiate an authentication
Reduces the time
The Physical Layer
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Direct Sequence Spread Spectrum (DSSS) PHY
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Indoor : IR : 1 and 2 Mbps
The OFDM PHY – IEEE 802.11a
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110KHz deviation : RF : PMD controls channel hopping : 2
Mbps
Infrared (IR) PHY
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2.4 GHz : RF : 1 – 2 Mbps
The Frequency Hopping Spread Spectrum (FHSS) PHY
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PLCP: frame exchange between the MAC and PHY
PMD: uses signal carrier and spread spectrum modulation to
transmit data frames over the media.
5.0 GHz : 6-54 Mbps :
High Rate DSSS PHY – IEEE 802.11b
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2.4 GHz : 5.5 Mbps – 11 Mbps :
IEEE 802.11E
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EDCF - Enhanced DCF
HCF - Hybrid Coordination Function
QBSS
HC – Hybrid Controller
TC – Traffic Categories
TXOP – Transmission Opportunity
 – granted by EDCF-TXOP or HC- poll TXOP
AIFS – Arbitration Interframe Space
IEEE 802.11E
IEEE 802.11E Backoff
IEEE 802.11 Protocols
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IEEE 802.11a
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IEEE 802.11b
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PHY Standard: 3 channels : OFDM and PBCC : 2nd half 2002
IEEE 802.11h
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Inter-Access Point Protocol : 2nd half 2002
IEEE 802.11g
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MAC Standard : QoS support : Second half of 2002.
IEEE 802.11f
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MAC Standard : operate in variable power levels : ongoing
IEEE 802.11e
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PHY Standard : 3 channels : 11 Mbps : Products are available.
IEEE 802.11d
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PHY Standard : 8 channels : 54 Mbps : Products are available.
Supplementary MAC Standard: TPC and DFS : 2nd half 2002
IEEE 802.11i
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Supplementary MAC Standard: Alternative WEP : 2nd half 2002
APPENDIX
The Basics of WLANs
Access speed
Range
PAN
LAN
WAN
1-2mb
11mb
>56kb
10m
100400m
global
IEEE
802.11b
GPRS
1xRTT
Low
device
specific
Medium
ethernet
High
regional
Infrastructure
FHSS
DSSS
cellular
Standard
Scalability
Architecture
WLAN Pending Issues
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Why 802.11a?
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Why 802.11b?
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Greater bandwidth (54Mb)
Less potential interference (5GHz)
More non-overlapping channels
Widely available
Greater range, lower power needs
Why 802.11g?
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Faster than 802.11b (24Mb vs 11Mb)
Deployment Issues
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Re-purpose Symbol AP’s for secure admin
services
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Deploy 802.11b with 802.11a in mind (25db
SNR for all service areas)
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Delay migration to 802.11a until dual
function (11b & 11a) cards become available
Frequency Bands- ISM
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Industrial, Scientific, and Medical (ISM) bands
Unlicensed, 22 MHz channel bandwidth
Short Wave Radio
AM Broadcast
Audio
FM Broadcast
Infrared wireless LAN
Television
Cellular (840MHz)
NPCS (1.9GHz)
Extremely Very Low Medium High Very Ultra Super Infrared Visible Ultra- X-Rays
Low
Low
High High High
Light violet
902 - 928 MHz
26 MHz
2.4 - 2.4835
GHz
83.5 MHz
(IEEE 802.11)
5 GHz
(IEEE 802.11)
HyperLAN
HyperLAN2
IEEE 802.11i Enhanced Security
Description
Enhancements to the 802.11 MAC standard to increase
the security; addresses new encryption methods and
upper layer authentication
Importance
High: weakness of WEP encryption is damaging the
802.11 standard perception in the market
Related
standards
This applies to 802.11b, 802.11a and 802.11g systems.
802.1x is key reference for upper layer authentication
Status +
Roadmap
Enhanced encryption software will replace WEP
software; This is on a recommended best practice
/voluntary basis; development in TgI: first draft Mar 2001;
next draft due Mar 2002; stable draft: July 2002; final
standard: Jan 2003
Products
affected
Client and AP cards (Controller chip, Firmware, Driver)
AP kernel, RG kernel, BG kernel
Agere’s activity
Actively proposing WEP improvement methods,
participating in all official/interim meetings
Key players
Agere/Microsoft/Agere/Cisco/Atheros/Intel/3Com/Intersil/
Symbol/Certicom/RSA/Funk
Key issues
Mode of AES to use for encryption (CTR/CBC [CBC MIC]
or OCB [MIC and Encryption function])
IEEE 802.1X - Port Based
Control
Description
A framework for regulating access control of client stations
to a network via the use of extensible authentication
methods
Importance
High: forms a key part of the important 802.11i proposals for
enhanced security
Related
standards
This applies to 802.11b, 802.11a and 802.11g systems
Status +
Roadmap
Standard available – Spring 2001
Products affected Supported in AP-2000, AP-1000/500, Clients (MS drivers for
XP/2000 beta)
Agere’s activity
Adding EAP auth types to products
Key players
Microsoft/Cisco/Certicom/RSA/Funk
Key issues
Home in IETF for EAP method discussions
IEEE 802.1p - Traffic Class
Reference
IEEE 802.1p (Traffic Class and Dynamic Multicast Filtering)
Description
A method to differentiate traffic streams in priotity classes in
support of quality of service offering
Importance
Medium: forms a key part of the 802.11e proposals for QoS
at the MAC level
Related
standards
This applies to 802.11b, 802.11a and 802.11g systems; is an
addition to the 802.1d Bridge standard (annex H).
Status +
Roadmap
Final standard; incorporated in 1998 edition of 802.1d
(annex H)
Products affected Client and AP cards (Driver); AP kernel, RG kernel, BG
kernel
Agere’s activity
Investigating implementation options
Key players
N/A
Key issues
N/A
Glossary of 802.11 Wireless
Terms, cont.
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BSSID & ESSID: Data fields identifying a stations BSS
& ESS.
Clear Channel Assessment (CCA): A station function
used to determine when it is OK to transmit.
Association: A function that maps a station to an Access
Point.
MAC Service Data Unit (MSDU): Data Frame passed
between user & MAC.
MAC Protocol Data Unit (MPDU): Data Frame passed
between MAC & PHY.
PLCP Packet (PLCP_PDU): Data Packet passed from
PHY to PHY over the Wireless Medium.
Overview, 802.11 Architecture
ESS
Existing
Wired LAN
AP
STA
BSS
AP
STA
STA
BSS
STA
Infrastructure
Network
STA
Ad Hoc
Network
STA
BSS
BSS
STA
STA
Ad Hoc
Network
Frequency Hopping and Direct
Sequence Spread Spectrum
Techniques
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Spread Spectrum used to avoid interference from
licensed and other non-licensed users, and from noise,
e.g., microwave ovens
Frequency Hopping (FHSS)
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Using one of 78 hop sequences, hop to a new 1MHz channel
(out of the total of 79 channels) at least every 400milliseconds
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Requires hop acquisition and synchronization
Hops away from interference
Direct Sequence (DSSS)
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Using one of 11 overlapping channels, multiply the data by an 11bit number to spread the 1M-symbol/sec data over 11MHz
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Requires RF linearity over 11MHz
Spreading yields processing gain at receiver
Less immune to interference
802.11 Physical Layer
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Preamble Sync, 16-bit Start Frame Delimiter, PLCP Header
including 16-bit Header CRC, MPDU, 32-bit CRC
FHSS
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2 & 4GFSK
Data Whitening for Bias Suppression
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32/33 bit stuffing and block inversion
7-bit LFSR scrambler
80-bit Preamble Sync pattern
32-bit Header
DSSS
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DBPSK & DQPSK
Data Scrambling using 8-bit LFSR
128-bit Preamble Sync pattern
48-bit Header
802.11 Physical Layer, cont.
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Antenna Diversity
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Multipath fading a signal can inhibit reception
Multiple antennas can significantly minimize
Spacial Separation of Orthoganality
Choose Antenna during Preamble Sync pattern
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Presence of Preamble Sync pattern
Presence of energy
•
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RSSI - Received Signal Strength Indication
Combination of both
Clear Channel Assessment
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Require reliable indication that channel is in use to defer
transmission
Use same mechanisms as for Antenna Diversity
Use NAV information
Performance, Theoretical
Maximum Throughput
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Throughput numbers in Mbits/sec:
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Assumes 100ms beacon interval, RTS, CTS used,
no collision
Mbit/sec
Slide courtesy of1 Matt
Fischer, AMD 2 Mbit/sec
MSDU size
(bytes)
128
DS
FH (400ms
DS
0.364
0.364
0.517
0.474
512
0.694
0.679
1.163
1.088
512
0.503
0.512
0.781
0.759
0.906
0.860
1.720
1.624
hop time)
FH (400ms
hop time)
(frag size = 128)
2304